Apparatus for igniting fuels



w J- WxMUITLEN Il APPARATUS FOR IGNITING FUELS Jan. 13, 1959 2 Sheets-Sheet 1 INVENTOR.

James W. Mu||en,I[

Filed April 29, 1946 ATTORNEYS.

Jan. 13, 1959 J. w. MULLEN n APPARATUS FOR IGNITING FUELS 2 Sheets-Sheet 2 Filed April 29, 1946 INVENTOR. James W. MuIlen,1I

ATTORNEx.

Wm, mm. Mv

United States Patent APPARATUS FOR IGNITIN G FUELS James W. Mullen II, Richmond, Va., assignor to Experiment Incorporated, Richmond, Va., a corporation of Virginia Application April 29, 1946, Serial No. 665,878

19 Claims. (Cl. 60-35.6)

.My invention relates to improved apparatus for the ignition of fuels and, in particular, to the application of such improvements as aids in the eicient combustion of fuels in ram-jets.

Even with a high-compression diffuser, the air-ow velocities within the combustion chamber of a ram-jet in supersonic ight are so great as render difficult the smooth combustion of the more desirable fuels (i. e. fuels having high heat release). Since the ram-jet depends for its thrust upon the change in momentum imparted (by combustion) to the intake air and to the fuel consumedit is of prime importance that the utmost available heat be released within the combustion chamber. Rough or explosive burning represents inefficient use of the available heat energy, with resultant loss of thrust and length limitations merely add to the diflculty of lt is another object to provide a means for supporting smooth combustion and for reducing the possibility of detonating air-fuel mixtures in a ram-jet.

It is also an object to provide an improved means for injecting fuel in a ram-jet.

Another object is to provide an improved means for etliciently utilizing a short combustion chamber in a ram-jet.

It is a further object to provide an improved means for burning fuel in a ram-jet with a minimum consumption of pilot fuel. l

Other objects and various further features of the invention will be hereinafter set forth or will become apparent to those skilled in the art from a reading of the following specification in conjunction with the drawings, in which:

Fig. 1 is a partially sectioned side elevation of a ramjet engine having an igniter incorporating features of the invention;

Fig. 2 is a fragmentary View in partial section showing elements of an injector-igniter according to the invention;

Fig. 2A is a fragmentary view in partial section illustrating elements of a modified injector-igniter;

Fig. 3 is a partially sectioned view showing an application to a larger structure of the principles of the apparatus of Fig. 2; and

` Fig. 4 is a sectional view taken essentially in the plane 4-4 of Fig. 3.

hf In early ground experiments concerned with the combustion of fuels under simulated ram-jet conditions, small pipes of one-inch diameter were employed as combustion chambers. At the time of conducting the experiments, the performance of these small burners seemed to vary between wide limits, ranging from very. smooth to erratic and rough burning. However, by comparison with later experiments in larger diameter facilities, very rough burning was seldom, if ever, achieved in the smaller Patented Jan.`13, 1959 ICCv installation. On the other hand, combustion in the larger pipes was often accompanied by violent detonations, in-

cluding detonatons upstream from the point of ignition. In an attempt to rationalize the difference in performance between the two sizes of burners, it was believed that the greater latitude of smooth burning characteristic of the smaller pipe could perhaps be correlated with the relatively short radial travel necessary for flame propagation from the ignition center to the combustionchamber wall, since it seemed reasonable that inherent flame instability apparent for large propagation distances provides the stimulus for detonating the relatively large volumes of uncombusted mixture. On4 these assumptions, it seemed that smooth combustion could be provided in the larger pipe by subdividing the combustion chamber into a series of successive stages; alternatively, the volumes of detonable mixture may be eliminated.

In principle, the process of subdivision involves isolating a first small fraction of combustible mixture, and successfully supporting combustion within that fraction before proceeding to ignite a second and larger fraction. The process is repeated for successive stages until continuous combustion of the entire flow of air-fuel mixture proceeds. In the specific form to be described, this multistage ignition takes place within a series of concentric shrouds, each entraining a progressively larger fraction of combustible mixture. The locus of the down stream ends of these shrouds is preferably upon the surface'of a geometric cone having essentially the same angle of divergence as a mechanically uninuenced tiame cone. Indeed, it has been found that under certain conditions a wider cone angle may be employed to locate the downstream ends of the shrouds.

To illustrate a specic form of multistage igniter, Fig. 1 shows an arrangement which has been successfully ernployed in ight. In this arrangement, the thrust produced by smooth combustion was sufficient to maintain the launched supersonic ight velocity.

The engine'to which the multistage igniter is shown applied is of the so-called annular-body type, comprising an annular nose section 5 providing a generally cylindrical opening 6 for the scooping of intake air. After passing through inlet duct 6, the intake air is expanded through a diffuser 7 to the full internal diameter of combustion -chamber 8. As a partial flow restriction, and to reduce the margin of thrust over drag at the launching velocity of the particular engine shown, a small reduction in diameter characterizes the exit 9 of the combustion chamber. It should be noted that this detail of construction may be included primarily for the purposes indicated, for subsequent tests have indicated that a reduced mass-ow is not a necessary condition for the smooth operation of my multistage igniter.

To conserve space, the multistage igniter is preferably mounted at the upstream end of the combustion chamber, that is, at the base of the diffuser. In the form shown,

i this mounting is accomplished by means of four streamlined struts 10 supporting a dare-holder 11, containing a pyrotechnic heat source 11', Which is employed as the center of ignition. Fuel is carried in an annular tank 12 surrounding the diffuser 7, and injected via a centrally located pipe 13 just downstream from the diffuser throat. Combustible mixture is thus presented to the upstream end of the combustion chamber, where a rst fraction of the total ow is isolated by the generally annular scoop presented by the end of a cylindrical shroud 14, which extends coaxially with and downstream from the center of ignition provided by flare 11. A second and larger fraction of the mass-flow is isolated by a second cylindrical shroud 15 concentric with shroud 14. Shroud 15 extends still further downstream. The downstream ends of, shrouds 14 and 15, together with the center of ignition,

preferably lie in a conical geometric surface living a divergence angle approximately that of a fiame cone under the anticipated burning conditions.

It will be observed that the igniter structure comprising the ignition center or flare 11 and shrouds 14 and 15 permits the gradual build-up of combustion in a plurality of stages-the first stage occurring lfor flame development from are 11 to the wall of shroud 14, the second occurring between the products of combustion Wit-hin shroud 14 and extending to the wall of shroud 15,- and the last occurring from the combustion products within shroud 15 and extending to the inner wall of the combustion chamber 8. rI`his combustion build-up proceeds from ignition of a small fraction of the total mass-flow, to a larger fraction, and finally to the remaining and largest fraction. Due to difficulties in and the novelty of present experimental techniques, no full explanation of the combustion process has yet been evolved, but it is thought likely that stability of the developing flame is assisted by the relatively intense heating of the downstream ends of shrouds 14 and 15 and by the transfer of heat from these ends of the shrouds to the uncombu'sted air-fuel mixtures flowing past their outer surfaces, thus effectingv improved vapo-riz'ation (and, hence, homogeneity of the mixture)l immediately prior to introduction of such mixture into the expanding ame.

Alternatively, the first stage of ignition may be viewed as a source 'of the chain carriers or intermediate unstable products `occurring in the combustion process. The heat released in this first stage imparts such increased velocity to the flow that substantial shearing action takes place between the exhaust jet and the adjacent annular volume of relatively low Velocity, uncombusted mixture. This shearing action may promote diffusion of the chain-carriers and of heat energy into the adjacent uncombusted mixture, withl resultant faster and better combustion of the latter. VThe same lanalysis may be applied for succeeding stages-etnie chain-carriers and heat developed in the second stage 'diffusing lreadily into th'e adjacent uncombusted mixture 'for Iimproved vfunction of the third stage of ignition, and the chain carriers and heat developed in the last stage promoting efficient combustion of the remaining uncombusted mixture. Thus, itfwill be noted that the increasingly larger requirements of heat and material transfer (for improved ignition) from one stage to its succeeding stage are self-providedby the structure described. It will be appreciated that this tendency to better the speed with which complete combustion takes place represents an artificial widening of the name-spread angle in excess of that occurring for a mechanically unimpeded fiow. With this artificially widened angle of spread, it is clear that shorter combustion chambers Imay be designed. l

lt has been observed, based on 'combustion experiments within tubular pipes simulating condition of ram-jet burners, that a heterogeneous -fuel feed Lis more conducive to quiet burning than is a homogeneous feed, 4but Vthat this process is in general the slower, requiring excessive lengths of combustion chamber. Presumably, this greater length is required to provide 'suieierrt `tiiii'e jfor the evaporation 'or air-mixing of the Yrelatively large droplets constituting the heterogeneous feedall prior 'to Eestablishment of anything approaching c'cJ'in'plete combustion "of the mixture. On the other hand, Vif 'all the'fuel has VAbeen so thoroughly mixed wit-h the intake lair Ias to provide a homogeneous mixture yat the point of ignition, v rough burning frequently results. This rough burning-is lthought to be attributable to detonation's upstream vof the fp' `nt of ignition, with resultant-interruption `of -fthe Lotherwise smooth -flow of combustible mixture `'past the point of ignition.

Broadly speaking, accordingto one feature of the-present invention, I seek to utilize a combination of these principles of fuel 'injectiomwhich combination 's 'thought to be a vpractical compromise of y'theffea'tties yof both, vIn

'4 other words, I provide means for presenting, at the point of ignition, a homogeneous mixture of such low fuel concentration that it is not detonable and for adding near the point of ignition or in the developing flame, the remainder of the fuel in the form of a heterogeneous mixture.

Fig. 2 illustrates a combustion chamber in which it has been fo-und possible to utilize the above-described homogeneous-heterogeneous mixture and to employ the shroud technique discussed for the case of the ram-jet of Fig. 1. In the apparatus of Fig. 2 the ow is` from left to right, and the elements' to be described are preferably located at the upstream end of the combustion chamber, i. e. at the base of the diffuser (not shown).

The arrangement of Fig. 2 is a combined injector-igniter having a plurality of stages 16, 17, and 18, preceded by a small piloting cone 19. Because of their ready availability, oxygen and hydrogen may be employed for piloting. Preferably, these gases are introduced separately into pilot 19 through pipes 20 and 21, for mixture in the turbulent region within the piloting cone; intermittent arcing from an insulated spark electrode 22 to the downstream skirt of the first igniter stage is adequate to assure initiation of combustion. In practice, once the main mixture of air and fuel is burning it has been found that the sparks from 4electrode 22 and the pilot fuel supplied through pipes 720 and 21 are no longer necessary; the piloting cone 19 then serves as a center for self-piloting of the entire igniter system.

As indicated above, tne multiple-stage injector-igniter of Fig. 2 includes means for injecting at each stage a fraction of the 'ram-jet fuel flow. In the case of the first stage 16, fuel is admitted through a pipe 23, which also serves to support the assembly of the pilot 19 and of the first stage or shroud 16. Fuel is then conducted to an annular passage 24 forming a manifold for the uniform distribution 'of fuel to a plurality of 'radially directed fuel-injector openings 25, located preferably in proximity to the 'downstream end of the pilot 19t It will be observed that the internal contour of the nist igniter stage 16 is such as to admit only a relatively sinall fraction of ythe total air mass-flow at the annular inlet opening 26 and that, just downstream from the injector openings 25, the expanding cross-section of the first stage 16 provides a region of relatively low flow velocity. Almost ideal conditions are thus presented for the successful mixing and smooth ignition of the first fraction of the total mass-flow.

.T he second stage or shroud 17 is constructed generally similar to the first stage 16, and it consists essentially of an 'annular shroud iin which there is an annular passage 27 'for the manifolding of fuel. Again, the assembly is supported by pipes 28 which serve for the admission of fuel to 'the manifold. For 'a purpose which will appear below, fuel is injected at the second stage 17 through inject-ion openings 29 directed upstream and 'preferably into or adjacent to the iiatne developing from the preceding stage. v

By virtue of the 'symmetrical disposition ofthe injector openings -29 with respect to vthe generally streamlined shape of the annulus comprising the fuel-injection stage 17, "some fuel droplets ma'y be entrainei 'within the central opening of 'the annulus, while others'run along the outer surface. Thus, 'there is an opportunity for the hot exhaust products from the first stage 16 to cause quick evaporation and combustion of some of the fuel injected at the Vsecond stage l17. As to the `uncombusted remainder of the fuel injected at this stage, which fuel may run down the inner or the outer wall ofthe shroud 17, a portion will bcdrawn'into a fish-tail or 4low-velocity turbulent region 30 lat the downstream end of the second "stage of ignition. This annular 'turbulent region 'may clearly serve as a second flame holder for stabilizing the flame developed b'y the second incremental mixture/of air and fuel. It will be noted A'that since 'the injection-ignition stage 17 spans a greater eross-sec't'ional 'area ythan 'the inlet '26 'to the anemie first stage 16, a greater fraction of the total air mass-flow is introduced to the combustion process at this stage, and that ignition of this greater mass-how may be stabilized at the downstream end of the second stage 17.

Exhausting from the second stage 17 is another expanding llame, which is permitted to impinge upon the third, and in this form the last, stage of ignition 18.` Structurally, the third stage or shroud 18 will be recognized as similar to the second stage 17. However, since a much larger fraction of the total air mass-ow is to be combusted, and since a correspondinglyv greater fraction of the total fuel flow is to be mixed with such air, the incremental cross-sectional area subtended by the third stage is much larger. Fuel is'admitted through pipes 31 (which also serve to support the structure) into an annular passage or manifold 32, and a plurality of injector openings 33 direct this fuel against the uncombusted air,in proximity to the expanding exhaust products from the second stage 17. An annular gutter or turbulent region 34 at the downstream end of the third stagey 18 serves to anchor at this region combustion of the remaining uncombusted gases.

Preferably, the annular space between the outer surface of the final stage 18 and the inner surface of the combustion chamber is relatively short. Such an arrangement is considered to favor smooth burning, for in use rough burning has seldom been observed as the main body of the ame expands from gutter 34 to the combustion chamber wall. j

It will be noted that the design of 4the several stages of ignition 16, 17, and 18 is such as to entrain successively greater fractions of the total air mass-how; in the form thus far described, it is preferable that the quantities of fuel admitted at these stages be so controlled that the airfuel ratio at each succeeding stage is substantially the same. In an alternative employment of the elements of Fig. 2, and illustrated in Fig. 2A, a fraction of the total fuel flow may be injected at an upstream location (as at 13 near the throat of the diffuser, asin Fig. 1) whereby a substantially homogeneous but non-detonable mixture is presented to the injector-igniter elements which have been described; in this alternative case also, it is preferable that the quantities of fuel injected at the successive stages be such as to maintain substantially the same air-fuel ratio in the air-fuel mass subjected to combustion at each stage.

Recalling discussions on the nature of multistage interaction, a brief rationalization may be helpful to an appreciation of the processes occurring from the abovedescribed alternative apparatus. In this explanation, the ratio of the total air mass-How to the entire mass-flow of ram-jet fuel will be referred to as the correct' air-fuel ratio.

According to the first described apparatus, a first small fraction ofthe total air mass-flow is entrained within the annular opening 26 in the first stage 16, and the quantity of fuel injected through openings 2S into this fraction is preferably suicient to provide substantially the correct air-fuel ratio within the remaining (downstream) volume of stage 16. Immediately afte'r leaving the stage 16 further quantities of air are introducedinto the region of the expanding flame, with the result that the combusting mixture becomes more lean, and at the same time physically more homogeneous. However, due largely to the absence of sharp delineations between burned and unburned mixtures within the region upstream from the second stage 17, the mixture is non-detonable although approaching homogeneity. To this homogeneous but non-detonable mixture there are added at stage 17 sufficient quantities of fuel to provide substantially the correct air-fuel ratio within at least the downstream volume of stage 17 and also (due principally to fuel running along the outer wall or surface of stage 17 )throughout a portion of the volume of the flame expandingjtherefrom. Downstream from this second region of substantially correct air-fuel ratio, a leaner mixture results from entrainrnent of a greater fraction of the air mass-flow into the developing .flame from the final stage expands to the combustionchamber wall. Thus, throughout the process of flame development there are provided substantially homogeneous, non-detonable buffer regions between successive stages; these regions may be considered as tending to inhibit detonative flash-backs between stages, with the result that more stable piloting zones may be maintained.

In the alternative apparatus, wherein a fraction of the fuel mass-how is introduced upstream from the igniter elements (as at 13 in Fig. 2A) to provide a substantially homogeneous and non-detonable mixture at the igniter, the supply of fuel at the various injection stages again approaches homogeneity only in the developing flame. Since the mixture cannot become detonable until after the fuel injected (at 25) near the downstream end of the pilot 19 has been substantially homogenized or mixed with non-detonably lean mixture entrained in the annular opening 26, there can, on the basis of the above argument, be no detonations back to the piloting cone. Conditions are thus favorable to steady burning within the pilot (i. e. to non-extinction of self-piloting) and to an essentially uninterrupted supply of combustible mixture to all stages of ignition.

Whether or not the above explanation may later prove to be correct, it is a fact that the elements of Fig. 2 have been operated both with and without upstream injection of a fraction of the fuel flow. Operation has been smooth and self-piloting, and has not been characterized by the violent detonations which occur in prior devices.

In Fig. 3 there is shown an application of the principles of the injector-igniter of Fig. 2 to a substantially larger structure in which more stages are provided to enhance smooth and efficient development of the flame. The rarn-` jet to which the igniter is shown adapted is of the so-called inner-body type, that is, one in which there is a centrally located, streamlined inner body 36 and in which the intake air is passed via the annular space between this body and the outer shell 35 of the ram-jet. Incidentally, the structure shown in Fig. 3 is one designed for a flying-type ramjet having a combustion chamber of 18 inches internal diameter.

In the broken-away arrangement of Fig. 3 the tail or downstream end of the inner body is shown at 36. The igniter structure is supported as by a threaded connection 37 to the inner body, which, conventionally, is fixedly spaced within the outer shell 35. Fuel to all injectors is supplied through a centrally located pipe 38. In the form shown, pilot ignition is provided by a cluster of socalled long-burning flares 39 nested within a container 40 and about the fuel pipe 38. 'y A The first small fraction of the total air mass-how is entrained into the first stage' of injection and of `ignition via a small annular opening 41 between the are casing 40 and a first shroud 42. Shroud 42 may be spaced from the are case 4t) by streamlined struts 43. In the form shown, the first small fraction of fuel is injected into the first small fraction of the air mass-ow through a series of injector nozzles 44 directing fuel transverse to the flow through the annular opening 41. Preferably, the shroud comprising the first igniter stage extends downstream from the pilot ignition means 39 and, during this extension, the cross-sectional area available for mixture and combustion is considerably expanded to provide a relatively low velocity turbulent region, as will be clear.

As in the case of the arrangement described for Fig. 2, the second stage 45 of the vinjector-igniter of Fig. 3 comprises a generally streamlined annular ring or shroud,

having an annular passage 46 for the manifolding of fuel to a plurality of, in this case upstream-directed, jets drilled or otherwise provided in the nose portion 47 of the ring. Since all lthe fuel is supplied via the pipe 38 'from the inner body 36, this member is also employed for support purposes; in the form shown, yfour hollow streamlined struts 48 are used. The downstream end or tail of the second stage 4S is preferably provided with an annular gutter 49 to serve as a flame holder for the expanding ilame developing as a result of the second stage of fuel injection and of air entrainment.

Succeeding stages or shrouds 50, 51, 52 are characterized by progressively greater diameters and by fuel injectors or openings having correspondingly greater capacity to direct fuel against the oncoming uncombusted air stream. In the form shown (see Fig. 4) the greater supply of fuel with succeeding stages is assisted by drilling correspondingly greater numbers 'of fuel-injection holes into the leading edges 53, 54, 55 of the injector rings. To complete the structure, the fuel-supply pipe 38 may be terminated with a streamline shape 56, and the entire structure may be rigidly supported centrally of the combustion chamber by means of support struts 57 carried by the ring of the last stage 52, as will be clear.

It is of interest that the structure which has been described for Figs. 3 and 4 has successfully operated in accordance with the principles indicated above. Smooth combustion can still be achieved after the flares have extinguished themselves, and of course under these conditions vthe low velocity region just inside vthe downstream end of the stage 42 provides a self-piloting zone.

It will be clear that I have described relatively simple apparatus for smoothly burning fuels in ram-jet combustion chambers. In accordance with these apparatus each successive stage of ignition or of lignition-injection serves as an anchor or pilot for the flame expanding downstream therefrom to 'the successive stage; in the case of the last stage, this piloting anchors the ame 'in its expansion to the .full internal diameter vof the combustion chamber. Once combustion has lbeen established in accordance with these methods, it is, possible to sustain smooth operation without the employment of pilot fuels, that is, without the employment of heat additives from sources other than the fuel.

lt is clear that the apparatus described provide .for such careful handling of the fuel that detonations upstream from any particular frame holder are improbable for the fuel or mixture admitted at that particular stage; burning must, therefore, be relatively smooth, even under unfavorable circumstances. Since the chances for rough burning are materially reduced over those prevailing for previous ignition methods, sustained higher combustion efliciencies are obtained, with the result that higher thrusts and specific impulses may be observed for the ram-jet. Since the burning is smooth, it is possible to design the downstream ends of the successive stages to fall on the surface of a geometric cone having a divergence angle substantially that of a developing flame; the methods and structures therefore lend themselves ,to utilization of relatively short combustion chambers.

While I have described preferred apparatus of the invention, it is to be understood that 'changes and .modifications may be made without departure from the scope of the invention as defined in the following claims.

I claim:

l. In a ram-jet, an air-inlet, a diffuser, a combustion chamber, an exhaust outlet, fuel-injection means for providing a substantially homogeneous air-fuel mixture in said combustion chamber, ignition means in the region of homogeneous air-fuel mixture, and further fuel-injection means in proximity to said ignition means.

2. vln a ram-jet, an inlet, a diffuser, a combustion chamber, an exhaust outlet, rst fuel-injection means vfor providing in .said combustion chamber an essentially homogeneous mixture with air entrained iby said inlet,

an ignitioncenter located downstream from said first fuel-injection means, second fuel-injection means in proximity to said ignition center, whereby said ignition center may initiate "combustion 'of a combined homogeneous and heterogeneous 'air-fuel mixture, and thirdv fuel-injection means for injecting fuel in proximity to v the products -of said combustion.

3. A ram-jet according to claim 2, in which said first fuel-injection means 'includes means limiting the fuel flow 'therethrough .to a 'quantity insufficient to make said homogeneous mixture 'detonab'le 4. In a ram-jet, a duct and injector and flame holder means within said duct and spaced therefrom, said means comprising a vhollow generally lannular shroud having a plurality of apertures in communication with the manifold formed by .the hollow of said shroud.

'5. ln a ram-jet, 'a duct having an upstream end and a downstream end, and linjector and flame holder means, including a generally annular shroud having an annular cavity forming a manifold, said shroud being of streamlined cross-section and having a plurality of apertures communicating between the cavity and the upstream edge of said shroud, said shroud also having a vortexforming discontinuity at its downstream edge.

6. A ram-jet device according to claim 5 in which the discontinuity at the Adownstream edge is of generally cupped section, whereby a locally turbulent flame-hold ing `region may be provided thereby.

r7. ln a ram-'jet injector and flame holder, an axially extending fuel-supply pipe, and a lplurality of annular fuel manifolds concentric with said pipe and in uidcommunicable relation therewith, said manifolds being axially spacedin the downstream direction in order of increasing diameter, said manifolds constituting substantially the only obstruction to flow when said injector and flame holder is mounted in the combustion chamber of a ram-jet, and said manifolds having apertures for substantially peripherally uniform injection of fuel.

8. vA ram-jet device according to claim 7 in which the apertures are of such size as 'to permit the injection fromv from successive manifolds of progressively greater frac-l tions of a given flow of fuel in said pipe, said fractions being generally proportional to the incremental crosssectional area subtended ybetween successive manifolds.

.10'. In a ram-jet injector and ame holder having a longitudinally extending axis, a plurality of hollow shrouds concentric with said axis, fuel supply means communicating with said shrouds, each of said shrouds having apertures communicating with its hollow interior, said shrouds being axially spaced in the downstream directionin order of .increasing diameter, and ignition means upstream of the trailing edge of the shroud of smallest diameter.

11. A ram-jet .device according to claim l0, wherein the shroud farthest upstream also extends upstream of 'said ignition means. y

1.2. .In a ram-jet, a duct with an inlet and an exhaust outlet, fuel-injection means within said duct, whereby with a flow of Aair in said duct there may be a mixture tiow downstream from said .fuel-injection means, longitudinally extending shroud means for mechanically isolating a fraction of .the total flow of said mixture, ignition means within said shroud means, `said shroud means extending downstream from said ignition means to a location at which 'substantial quantities of chain carriers will be generated Ib y combustion within said shroud means .for the design air lflow and fuel, whereby at the downstream kend of said shroud means the flow velocity within said vshroud means may exceed the flow velocity of the uncombusted remainder of the mixture, the downstream end of said shroud means being upstream from said exhaust outlet, whereby the products of combustion within said shroud means may be discharged to the uncombusted remainder of said mixture with a ow generally parallel to the ow of said uncombusted remainder, so that turbulence generated by shearing action due to the proximity of two flow velocities may be utilized for diffusion into said uncombusted remainder of chain carriers and of other products of combustion within said shroud means.

13. In a ram-jet, a duct with an inlet and with an exhaust outlet, rst fuel-injection means for injecting a rst and relatively small fraction of a total liow of fuel into an airstream through said duct, ignition means downstream from said injection means for igniting the resulting air-fuel mixture, whereby an expanding generally conical flame may be generated in a first stage of combustion, and second fuel-injection means for injecting a second and relatively large fraction of the total flow of fuel, said second injection means being located downstream from said ignition means and including means for the substantially circumferentially uniform injection of fuel into said duct about a generally circular configuration of diameter less than the minimum internal dimension of said duct at the location of second injection, said generally circular configuration being disposed substantially uniformly peripherally in a section taken through the expected expanding conical ame, whereby a generally conical llame may be further expanded `in a second stage of combustion.

14. In a ram-jet, a duct with an inlet and with an exhaust outlet, fuel-injection means, means for dividing the fuel flow, and means for igniting the divided ow of the fuel charge in successive longitudinally spaced stages, the locus of ignition at each such stage being generally circular and of an effective diameter increasing with the downstream spacing away from the location of rst ignition. v

15. In a ram-jet, a duct with an inlet and with an exhaustoutlet, fuel-injection means including means for introducing divided parts of the fuel charge at each of a succession of longitudinally spaced combustion stages, and means for igniting the divided parts of the fuel charge in saidA stages at longitudinally spaced generally radial planes, the locus of ignition at each such generally radial plane being essentially a continuous line in general conformity with the desired generally conical ame development in said duct.

16. A ram-jet according to claim 15, in which the fuel increments accommodated by said igniting means are greater at succeeding downstream locations.

17. In a ram-jet, a duct with an inlet and an exhaust outlet, ignition means, fuel-injection means including a plurality of shrouds within said duct and means for introducing divided increments of the injected fuel for successive of said shrouds, said shrouds being spaced from said ignition means and having successively larger downstream ends spaced downstream from said ignition means, whereby combustion may be effected in stages corresponding substantially to the division of fuel.

18. In a ram-jet having an axis of symmetry, an inlet, a diffuser, ignition means, a combustion chamber, and an exhaust outlet, all symmetrically disposed about said axis, and fuel-injection means including means for introducing divided increments of the injected fuel for localized combustion within said chamber, said lastnientioned means including a plurality of shrouds spaced from said ignition means and having portions located downstream therefrom and within said chamber, the downstream ends of said shrouds deiining with said ignition means an angle of divergence from said axis, said angle exceeding the half-angle of spread of the flame cone occurring for a mechanically unimpeded flow equal to the design internal mass-ow for the ram-jet.

19. ln a ram-jet, a duct including an inlet, a diifuser, a combustion chamber, and an exhaust outlet; fuel-injection means, ignition means downstream from said fuelinjection means, whereby with a ow of air in said duct there may be a mixture flow originating upstream of said ignition means, said ignition means including a plurality of longitudinally extending shrouds within said duct and of size increasing with the location of the downstream ends thereof, a downstream shroud including a part generally radially spaced from and longitudinally overlapping a part of an upstream shroud, and an igniter within the upstreamshroud, whereby said shrouds may serve to dene limits for dividing and isolating combustible charges of said mixture ow, and whereby with combustion originating within said upstream shroud a portion of the heat thereby developed may be transferred via said upstream shroud to preheat the isolated combustible charge ybetween said upstream and downstream shrouds, so that combustion products from within said upstream shroud may be discharged into an isolated and preheated charge.

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